The dermis is the median district of the skin which performs supporting and trophic functions and confers strength, elasticity, turgidity and metabolic viability on the skin as a whole. The dermis contains cells called fibroblasts, i.e. specialised cells of mesenchymal origin, which continuously regenerate the interstitial matrix mainly consisting of glycosaminoglycans (GAGS) and proteoglycans (PGs), hydrophilic substances of a glycoside and glycopeptide nature respectively. Said macromolecules retain large amounts of water, constituting a hydrophilic network in gel form in which all the fibrous proteins that give the skin its strength and tone are immersed.
The fibroblasts are also responsible for synthesis of elastin, a protein with elastic properties that gives the skin and mucous membranes the fundamental property of adapting to the morphological and mechanical changes they undergo.
Elastin is capable of stretching up to 7 times its own length and returning to its dimensional modulus without significant molecular alterations, and can theoretically repeat this stretching an unlimited number of times.
The conditions known as skin aging and photoaging represent the objective dermatological manifestations of highly complex biochemical phenomena involving the cells, the superficial tissue structures and, above all, the deep tissue structures.
Photoaging causes the appearance of wrinkles of varying numbers and depths on the skin surface, especially where it is exposed to light (face, neckline and torso), elastosis and more or less diffuse pigmented spots, epidermal thickening and freckles. The elastic component of the skin, represented by the network of elastin fibres located in the distal part of the dermal tissue (reticular dermis), undergoes significant alterations as a result of the photoaging phenomena described above, causing a significant loss of the total elastic capacity of the skin. At macroscopic level, said phenomena reduce the ability of the connective tissue of the skin to adapt to mechanical stretching, leading to tissue sagging and elastosis.
The gene encoding the protein tropoelastin (ELN-gene), which is the precursor of elastin, is unique; there are not several genes (superfamily) encoding different forms of elastin as is the case with collagen [1]. Said gene encodes different forms of tropoelastin (thus there is probably a different tropoelastin encoding process for healthy and newly-formed tissues, for example in response to physical damage such as burning or photoaging); it already begins to be expressed at the foetal stage, remaining active for the first 5 years of life and then slowing drastically, until its activity stops [1,2]. In other words, the elastic component of connective tissue, and in particular of the dermis, mucous membranes, cartilage tissue, tunica intima, pulmonary and valvular/myocardial connective tissue, already ceases to be synthesised in the early years of life. Said component consequently represents the body's “elasticity supply” and is never replenished during its lifetime, except in the event of serious tissue damage, such as burns and severe photoaging. In these cases there is over-expression of the LOX (lysine oxidase) genes, which encode 5 different enzymes that catalyse oxidation of the lysine residues in the tropoelastin precursor molecules, a necessary step for the synthesis of functional elastin and its subsequent incorporation in the microfibrils adhering to the cell surface [3,4]. In particular, said enzyme-dependent process consists of lysine oxidation and simultaneous formation of a Schiff base between the amino group of L-LYS and an aldose, to give rise to a crosslinked intramolecular bond [5,6]. Elastin is the only connective tissue protein that is substantially not replaced, and remains the same for over 70 years (mean half-life 74 years).
In the case of collagen, type IV collagen plays a key role in the process of structuring the basal dermoepidermal membrane and the supramolecular organisation of extracellular matrix (ECM), with special reference to cell orientation in the matrix. A collagen IV deficiency is closely correlated with loss of the trophism and elastic power of human skin, especially in the skin degeneration phenomena typical of skin aging and photoaging [7,8].
Specific combinations of amino acids and oligopeptides, if suitably carried and applied topically or orally, are known to promote the gene expression that gives rise to new protein synthesis in the dermoepidermal connective tissue, mucous membranes and joint cartilage, especially the synthesis of collagen and tropoelastin [9,10,11,12].
One of the main phenomena involved in the progressive decline of the elastic function of the dermoepidermal tissues in skin aging and photoaging is represented by progressive loss of biosynthesis activity by the fibroblasts, the cells responsible for regeneration of the collagen and elastin proteins and of the dermal extracellular matrix (ECM). As a result of this phenomenon, in the event of insufficient or low synthesis of new structural dermal proteins and hydrophilic extracellular matrix, their degradation by specific catabolic enzymes called elastase, metalloproteinase, collagenase and gelatinase increases [13,14,15]. Said enzymes, which are hyperexpressed and synthesised in the mitochondria of aging fibroblasts and the macrophages, shift the “synthesis-demolition” balance towards demolition, leading to a slow but progressive deterioration of the tissue, which thus becomes less compact, less elastic and less hydrated (deficiency of GAGs, PGs and fibroelastin component). To inhibit or at least limit said phenomenon, which is partly physiological but exacerbated by various environmental and genetic causes, it is necessary to promote the expression of genes that induce the synthesis of structural proteins in the fibroblasts (tropoelastin and collagen) in all areas wherein the latter are present, and at the same time reduce the, expression of genes responsible for encoding metalloproteinases, particularly those specialising in the degradation of collagen and elastin (collagenase and elastase).
In view of the increased catabolic activities mediated by gene hyperexpression of matrix-degrading enzymes, especially collagenase and elastase, topical or injective application, directly into the dennoepidermal tissue, mucous membranes or joints, of substances with proven activity inhibiting the expression of said enzymes, is particularly strategic. In particular, N-acetylcysteine, the N-acetylated form of the sulphurated amino acid L-cysteine, exhibits an evident action inhibiting the expression of metalloelastases induced by photo-irradiation [16]. Its simultaneous introduction into polyaminoacid compositions that perform an elastogenic and collagenic activity is advantageously usable to promote recovery and maintenance of the elastin and collagen content of the connective tissue.
Hyaluronic acid (HA) is the main GAG present in the amorphous interstitial matrix of connective tissue.
GAGs (glycosaminoglycans) are polysaccharide molecules consisting of repeating mono- or disaccharide units; hyaluronic acid is a polyglucodimer consisting of N-acetyl glucosamine and glucuronic acid. HA is the only GAG whose molecule does not include sulphate groups, and the only one with an unbranched linear structure. The other GAGs present in the amorphous matrix of connective tissue, and components of other connective tissue structures such as tendons and cartilage, are chondroitin sulphate, keratan sulphate, heparan sulphate, dermatan sulphate and heparin. HA performs crucial hydrating functions in ECM, promoting cell mobility, exchange of nutrients and soluble protein factors, and modulating the biochemical phenomena of regeneration and organisation of the fibro-connective tissue matrix.
Mixtures of amino acids for injectable, topical or oral use are disclosed in EP 2 033 689, WO 2007/048522 and WO 2011/064297. However, none of the compositions disclosed contains a mixture of glycine, L-proline, L-alanine, L-valine, L-leucine and L-lysine hydrochloride in suitable ratios, able to stimulate the synthesis of both collagen and elastin.